i) Field of the Invention
This invention relates to a nanocomposite of nanocrystalline cellulose (NCC) and polylactic acid (PLA), and a process for producing such a nanocomposite. The nanocomposite is a sustainable, thermally-stable biomaterial which is hydrophobic and compatible with a wide range of synthetic and natural polymers. NCC-PLA nanocomposites have enhanced mechanical performance and dimensional stability (i.e. less hygroexpansivity) relative to PLA, and are potentially biocompatible, recyclable and made from entirely renewable resources. NCC-PLA nanocomposites can be suspended in many organic solvents or dried to form a solid substance that can be processed using conventional polymer processing techniques to develop 3-dimensional structures.
ii) Description of the Prior Art
The 20th century has been characterized, amongst many things, as the plastics century. It is impossible to imagine the current world without plastics. Products range from baby bottles to packaging materials to components in aircraft. While polyolefins have been the basis of almost all commercial plastic development, questions have been raised about their long-term applications due to concerns over recycling, health and environmental responsibility—e.g., the need to use chemical building blocks, such as bisphenol A (BPA), in the manufacture of polycarbonate plastics and epoxy resins.
In response, serious efforts have been rendered to develop bioplastics and biomaterials made from renewable resources. Nanocrystalline cellulose (NCC) is highly crystalline cellulose extracted from woody or nonwoody biomass. Polylactic acid (PLA) is a thermoplastic, aliphatic polyester made up of lactic acid (2-hydroxy propionic acid) building blocks. PLA is derived from renewable plant sources, such as starch and sugar, and has potential applications in medicine, engineering and food and beverage packaging. The degradation of PLA can be achieved through the hydrolysis of the ester bonds without the need for enzymatic treatment. However, PLA has certain limitations compared to polyolefins, particularly during processing, which has greatly limited its use. PLA is essentially hygroscopic and possesses low thermal resistance. Nano-scale reinforcement can be used to enhance the rheological, mechanical and physical properties of polymers, thereby improving their processability, functionality and end-use performance. NCC can function as high performance reinforcement owing to its large specific surface area, high strength and high surface reactivity. To achieve this, two critical conditions are required: (i) excellent dispersion of the NCC within the polymer matrix, and (ii) perfect compatibility between the two in order to produce excellent interfacial mechanics in various environments.
Studies have shown that NCC can be used to increase the storage modulus of some nanocomposites by orders of magnitude above the glass transition temperature of the matrix [1-3]. By compounding NCC with PLA, it is possible to make a biomaterial, made from entirely renewable resources, with satisfactory properties. However, since NCC is hydrophilic and PLA hydrophobic, compatibility becomes a critical and difficult issue. Published results have indicated it is practically impossible to prepare NCC-PLA nanocomposites by directly compounding PEA and unmodified, or only physically modified using surfactants or polymer compatibilizers, NCC [4-8]. A more recent study demonstrated that the dispersion of NCC in PLA matrix can be improved by grafting NCC with polycaprolactone (PCL) [9]. However, aggregation of the nanoparticles was still observed in such a system and the improvement in the mechanical properties of the resulting nanocomposite was limited.